i think it takes time to figure that out. I can't base things on how a guy looks at me for 20 seconds. I need sometime to talk to them to access them. I guess I try to judge someone's character and personality within at least a few days of getting to know them.
depends on what room you are in , e.g class room ,it may be out of curiosity to see how is coming into the room or boredom.if she looks and smiles or winks , or keeps eye contact longer then usual , then maybe she's interested oh and if she follows you as you walk with her stare- although that does sound quite weird !
i don't think you can ever say!
i alwaysss check people around me :P I like to know who is coming or crossing in front of me...emm let me correct myself, not EVERYONE, but most of the people who looks good in far...but it doesn't mean I always have a good shot, sometimes I wonder how they look good when they are walking in distance :D
but for making sure that which reason is true, you need more information or catch her checking on you on other time or occasions ;)
I think it all depends on the extent of the look both in time and placement. If they simply glance up then away odds are their eyes were simply drawn by your movement. If they look up and stare you in the face for a couple seconds, they are trying to establish some kind of a communicative connection. If they glace over and look you up and down, they are making an analysis of you sexually. My opinion on the matter anyways.